This invention relates to pilot controlled valves and, in its presently preferred embodiments, to a new and improved pilot stage valve for use in spool-type valves and cartridge-type valves.
Spool-type valves are typically used to control the flow of fluid, such as hydraulic oil, water or air. The size and diameter of the spool determine the flow capacity of the valve. The position of the spool within its valve body controls the amoun and direction of fluid flow through the valve. Because the fluid flow forces and spool mass are typically high, pilot stage valves are used to control the spool position.
There are generally three types of pilot stages for spool-type valves: directional control, proportional control, and servo control. The directional control pilot valve is used to turn fluid flow on and off. This valve is used in the majority of applications. The proportional control pilot valve controls the amount of fluid flow through the valve. The use of these valves in applications is increasing. Servo-type pilot valves use mechanical feedback from the spool to the pilot stage to control spool position. These valves are used in high performance, proportional control applications.
One conventional type of directional spool valve uses a solenoid controlled pilot stage. A first solenoid and iron plunger are attached to one end of the valve housing, and a second solenoid and iron plunger are attached to the opposite side of the valve housing. The solenoids are alternately energized to move the spool and turn the fluid flow on and off. Specifically, when the first solenoid is energized it forces its iron plunger in a direction which moves the spool to turn on fluid flow. When the second solenoid is energized its iron plunger returns the spool to the off position.
This type of conventional directional valve has several drawbacks. The two solenoids and their associated electrical connections add bulk, weight, and significant power consumption to the valve package. The iron slugs are relatively heavy and thus require a lot of electrical energy to be moved by the solenoids. Twenty-four volts and one amp are typical electricity requirements, which computes to twenty-four watts of power. The solenoids also have a relatively long response time, generally around 100 milliseconds.
One type of conventional proportional valve also uses a solenoid controlled pilot stage. In this valve, however, a solenoid and its plunger are attached to only one end of the spool. A spring is attached to the other end of the spool. When the solenoid is energized it moves its plunger in a direction to push the spool against the bias of the spring. The force of the spring provides proportional control of the flow of fluid. When the solenoid is de-energized the spring forces the spool to the off position.
This type of proportional valve also shares the drawbacks of the solenoid controlled directional control pilot valve. Specifically, high electrical current, and thus power, is necessary to move the spool. Moreover, the spring force on the spool is not well suited for high pressure applications.
A widely used servo-type valve is disclosed in U.S. Pat. No. 3,023,782 (Chaves). This valve uses a torque motor pilot stage with negative feedback provided by a flapper 73 in mechanical contact with the spool. The flapper shifts the spool, which can be subject to large fluid forces in response to a small electrical signal to the torque motor. Thus, the flapper provides substantial fluid amplification. The position of the flapper is negatively fed back to the torque motor to control the spool position. This negative feedback provides linearity and minimizes hysteresis.
While the Chaves servo valve provides some advantages, it also has significant disadvantages. These valves are complex and expensive to make. The current price for a 10 gallon per minute (gpm) valve is around $1000.00. Furthermore, these valves are susceptible to clogging due to the small mechanical tolerances (on the order of 0.005 inch) of the flapper design. Thus, extensive filtering of hydraulic fluid, such as oil, is necessary to avoid contamination problems.
Another servo operated spool valve is disclosed in U.S. Pat. No. 3,106,224 (Moss). This patent discloses a spool 1 and a cylindrical spindle 13, which extends through an axial bore in the spool and the two ends of the valve housing 7. Two helical grooves 15 and 16 are formed in the surface of the spindle and are spaced from each other by approximately one-half helical pitch, so that each groove extends from one end of the valve housing cavity past a pair of diametrically opposed radial bores 17 in the spool. In its central position, the radial bores should be inside the central port 3 of the valve housing, and each groove should uncover equal parts of one of the radial bores.
The spool is maintained in its central position by a continuous flow of oil through the valve housing and spool that provides equal fluid pressure at both ends of the housing bore. In particular, the oil flows along two branches from the pressure inlet 24, through ports 2 and 4, passages 20 and 22, and orifices 21 and 23, to the two end chambers of the bore in housing 7, through the grooves 15 and 16 and the radial bores 17 to the drain port 12. In this central "null" position, lands 5 and 6 block the flow of oil through the service ports 10 and 11.
In order to move the spool axially, the spindle 13 is rotated. This will cause one groove to uncover a greater portion of one radial bore and the other groove to cover a greater portion of the opposite radial bore. As a result, the fluid pressure in one end chamber of the valve housing will be greater than the other, and the spool will move towards the chamber of lower pressure until the fluid pressure in each chamber is equal. At this point, each radial bore will be uncovered the same amount again. The axial movement of the spool is proportional to the rotary displacement of the spindle 13.
The Moss servo valve, at first blush, may appear to be less complex and more desirable than the Chaves servo valve. However, the Moss servo valve also has some significant drawbacks. The Moss valve is designed to have continuous oil flow, even at null, between both ends of the valve housing to balance the pressure across the spool. This continuous flow requirement complicates the design and manufacture of the valve. The spindle grooves 15 and 16 and radial bores 17 must be designed in a relationship that facilitates constant flow. The passages 20 and 22 and orifices 21 and 23 must be machined into the outer lands 18 and 21 of the spool. Moreover, the orifices 21 and 23 must be the same size so that each end chamber has about one-half of the fluid pressure at the null position. The orifices and radial holes should also be small to minimize flow at null. The small holes, however, are more prone to contamination.